Clinical and imaging characteristics of patients with axial psoriatic arthritis: Baseline findings from the GESPIC-axial-PsA cohort.

Background: Axial disease is a domain of psoriatic arthritis (PsA), referred to as axial-PsA. Despite recent advances and increasing recognition, axial-PsA remains poorly defined; validated classification criteria and longitudinal cohorts are lacking. Objectives: We aimed to characterise axial-PsA and delineate phenotypes in an imaging-confirmed cohort. Design: GESPIC-axPsA, a dedicated arm of the German Spondyloarthritis Inception Cohort (GESPIC), is a prospective, monocentric, study including patients with psoriasis and clinically diagnosed axial-PsA confirmed by imaging. Baseline assessments included clinical phenotyping, laboratory testing, biosample-collection, radiographs of the axial skeleton, and MRI of the whole spine and sacroiliac joints (SIJs). Methods: The analysis was primarily descriptive, characterizing baseline clinical and imaging features of patients with imaging-confirmed axial-PsA; categorical and continuous variables were summarized using appropriate measures based on their distribution, and subgroup comparisons were performed using chi-square or Fisher's exact tests for categorical variables and Mann-Whitney U tests for continuous variables after assessing normality. Results: We enrolled 107 axial-PsA patients (55% female; mean age 44.8 years); 78.5% had active psoriasis, 78.6% had inflammatory back pain (IBP), and 48.6% were HLA-B27 positive. Radiographic sacroiliitis was present in 45.7%. On MRI, active/structural SIJ lesions were seen in 53.3%/73.3%; spinal active/structural lesions in 60.0%/53.3%. Both Classification criteria for axSpA (ASAS) and PsA (CASPAR) were simultaneously fulfilled in 60.7%. We identified 20 patients (18.7%) with spine-only involvement-spinal MRI lesions without SIJ lesions on radiography or MRI. Compared with the remainder, the spine-only group was older, more often female, less frequently HLA-B27-positive, and had back pain less consistent with IBP (all p<0.05). Conclusions: In our imaging-confirmed axial-PsA cohort using standardized whole-spine and SIJ MRI, we observed marked phenotypic heterogeneity. Approximately one fifth had isolated spinal involvement; this subgroup differed by age, sex, HLA-B27, and IBP features, with imaging distributions distinct from axSpA-supporting the need for disease-specific classification criteria.